VBAについて教えて！なんて声をちらほらと聞きまして。
ということで、日付型変数を操作する自作の便利関数を紹介。

ご自由にお使い下さい。↓↓↓

'--------------------------------------------------------
'YYYYMMDD形式の文字列変数を日付型変数に変換する関数
'--------------------------------------------------------
Public Function ToDateFromYMD(Val_Str As String) As Date
Dim Val_Year, Val_Month, Val_Day As Integer

Val_Year = Left(Val_Str, 4) '[YYYY]を抽出
Val_Month = Left(Right(Val_Str, 4), 2) '[MM]を抽出
Val_Day = Right(Val_Str, 2) '[DD]を抽出

ToDateFromYMD = DateSerial(Val_Year, Val_Month, Val_Day)
End Function

'--------------------------------------------------------
'指定された日付をYYYYMMDD形式の文字列に変換する関数
'--------------------------------------------------------
Public Function ToYMDDate(Val_Date As Date) As String
Dim Val_Year, Val_Month, Val_Day As Integer

Val_Year = Year(Val_Date) '[YYYY]を抽出
Val_Month = Month(Val_Date) '[MM]を抽出
Val_Day = Day(Val_Date) '[DD]を抽出

ToYMDDate = Val_Year & Val_Month & Val_Day
End Function

'--------------------------------------------------------
'指定された日付の月の初日を返す関数
'--------------------------------------------------------
Public Function GetFirstDate(Val_Date As Date) As Date
Dim Val_Year, Val_Month, Val_Day As Integer

Val_Year = Year(Val_Date) '[YYYY]を抽出
Val_Month = Month(Val_Date) '[MM]を抽出
Val_Day = 1 '[DD]を指定

GetFirstDate = DateSerial(Val_Year, Val_Month, Val_Day)
End Function

'--------------------------------------------------------
'指定された日付の月末日付を返す関数
'--------------------------------------------------------
Public Function GetLastDate(Val_Date As Date) As Date
Dim Val_NextDate, Val_LastDate As Date

Val_NextDate = DateAdd("m", 1, GetFirstDate(Val_Date)) 'Val_Dateの翌月の1日日付を格納
Val_LastDate = DateAdd("d", -1, Val_NextDate ) 'Val_Dateの翌月の1日日付から1日減算

GetLastDate = Val_LastDate
End Function

'--------------------------------------------------------
'日付から曜日を返す関数
'--------------------------------------------------------
Public Function GetWeekday(Val_Date As Date) As String
Select Case Weekday(Val_Date)
Case 1
GetWeekday = "日"
Case 2
GetWeekday = "月"
Case 3
GetWeekday = "火"
Case 4
GetWeekday = "水"
Case 5
GetWeekday = "木"
Case 6
GetWeekday = "金"
Case 7
GetWeekday = "土"
End Select
End Function